A conventional in-wheel motor drive unit is disclosed in Japanese Unexamined Patent Publication No. 2001-32914, for example. The in-wheel motor drive unit disclosed in this document comprises a motor generating driving force, a decelerator decelerating the rotation of the motor and transmitting it to a drive wheel, and a wheel hub rotatably retaining the drive wheel.
The decelerator includes a planet gear mechanism comprising a sun gear provided in an inlet shaft, an internal gear fixed to a casing, and a planet gear arranged between the sun gear and the internal gear, and connected to the output shaft. In addition, the two planet gear mechanisms are arranged in series to enhance a speed reduction ratio.
The wheel hub is fixedly connected to the output shaft of the decelerator and rotatably supported by a wheel hub bearing with respect to the casing. The wheel hub bearing is a double-row rolling bearing comprising an inner ring fit in the outer diameter surface of the wheel hub, an outer ring fit in the inner diameter surface of the casing, a plurality of rolling elements arranged between the inner ring and the outer ring, and a retainer retaining the plurality of rolling elements.
It is reported that an electric car having the above in-wheel motor drive unit has the advantage that an effective space is increased in the car since it is not necessary to ensure a space for a drive unit in a car body, and there is no efficiency degradation and no weight increase due to a transmission system such as a differential unit.
In addition, a conventional in-wheel motor drive unit is also disclosed in Japanese Unexamined Patent Publication No. 2005-7914, for example. The in-wheel motor drive unit disclosed in this document comprises a motor generating driving force, a wheel hub connected to a tire, and a decelerator decelerating the rotation of a rotor of the motor and transmitting it to the tire. This decelerator employs a parallel shaft gear mechanism in which a plurality of gears having the different number of gear tooth are combined.
Since the in-wheel motor drive unit in which the output shaft of the electric motor and the hub of the wheel are connected coaxially through the decelerator does not need a large power transmission mechanism such as a propeller shaft and a differential, the car can be light in weight and compact in size. However, the in-wheel motor drive unit mounted under the spring of the car has a problem that its ride is uncomfortable due to the increase in unsprung weight, so that it has not been practically used yet.
Since the output torque of the electric motor is roughly in proportional to the motor capacity (weight), the electric motor has to be rotated at high speed to obtain a sufficient output required to drive the wheel of the car with small motor capacity, so that it is necessary to incorporate a decelerator between the output shaft of the electric motor and the hub. In this case, since the decelerator to be incorporated in the in-wheel motor drive unit has to be light in weight, the decelerator is required to be compact in size and have a high speed reduction ratio.
In addition, one deceleration unit for the electric car incorporates a planet gear decelerator as a decelerator between the output shaft of the electric motor and the hub of the wheel (refer to Japanese Unexamined Patent Publication No. 5-332401). According to the one disclosed in the above embodiment, although it is not the in-wheel motor drive unit in which the electric motor and the decelerator are mounted under the spring, first and second planet gear decelerators are provided and the output from the second planet gear decelerator is distributed to the right and left wheels under the spring, through a drive shaft.
Since the above in-wheel motor drive unit is arranged under a suspension, the problem is that running stability is lowered due to the increase in unsprung weight. This problem becomes serious as the recent car becomes compact as a whole.
In addition, according to the wheel hub bearing, since the inner ring and outer ring are arranged between the wheel hub and the casing, the problem is that the radial dimension is increased. Furthermore, since the number of components is large, the assembling property is not preferable.
In addition, according to the speed reduction ratios of the parallel gear mechanism and the planet gear mechanism employed in the decelerator in both documents, the former is set to 1/2 to 1/3, and the latter is set to 1/3 to 1/6 in general. Since this is not sufficient as the speed reduction ratio of the decelerator mounted on the in-wheel motor drive unit, it is necessary to provide a multiple-stage decelerator to obtain an enough speed reduction ratio. However, this causes an increase in weight and size of the decelerator, which is not suitable for the in-wheel motor drive unit required to be compact in size.
In addition, although the planet gear decelerator has a high speed reduction ratio as compared with the parallel gear, since the planet gear decelerator comprises a sun gear, a ring gear, a pinion gear and a carrier of the pinion gear, it cannot be compact because the components are large in number.